Pregnancy due date calculator
Estimated due date and current gestational age from the last menstrual period.
Leave at 28 unless the cycle is reliably different.
Optional. Defaults to today.
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This is a reference implementation of a published formula, provided for use by qualified healthcare professionals. It does not diagnose, does not account for the individual circumstances of any patient, and is not a recommendation for treatment. Confirm every result against your own institutional guidance before acting on it. Clinical judgement always takes precedence over a calculator.
About this calculation
Naegele’s rule estimates the due date as 280 days from the first day of the last menstrual period. It assumes a regular 28-day cycle with ovulation on day 14; a cycle-length adjustment is offered because that assumption frequently does not hold.
Formula
EDD = first day of LMP + 280 days, adjusted by (cycle length − 28) days
- Source
- Naegele’s rule; ACOG Committee Opinion 700 on estimating due date (2017)
- Methodology version
- 1.0.0
- Last reviewed
- 2026-10-28
What this does not account for
- An LMP-based date assumes a regular cycle and accurate recall of its first day. Both are frequently wrong.
- First-trimester ultrasound dating is more accurate and, where the two differ by more than the accepted margin, replaces the LMP date.
- The estimate does not apply to a pregnancy conceived through assisted reproduction, where the transfer date is used instead.
- Only around 4% of births occur on the estimated date. It is the centre of a distribution, not a prediction.